Stock-based Compensation - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based compensation expense
Stock-based compensation expense	$ 22,759	$ 13,489	$ 3,254
Research and development
Stock-based compensation expense
Stock-based compensation expense	8,277	3,707	883
General and administrative
Stock-based compensation expense
Stock-based compensation expense	14,482	9,782	2,371
Stock options
Stock-based compensation expense
Stock-based compensation expense	22,487	12,997	2,662
Unvested restricted common stock
Stock-based compensation expense
Stock-based compensation expense	$ 272	$ 492	$ 592